Variable Gain Amplifier

ABSTRACT

A method of varying the gain of an amplifier and an amplifier array are provided. The amplifier array includes two or more amplifier stages ( 201, 202 ) connected in parallel with each amplifier stage having a gain control means. Input signal means ( 203, 204 ) are provided for each amplifier stage with the input signals of the amplifier stages being of different amplitude. Means for enabling and disabling an amplifier stage ( 216 ) are provided and means for summing the outputs of the enabled amplifier stages obtain an output signal ( 212 ). The gain of the amplifier array has a range from a low gain setting with a first amplifier stage ( 202 ) enabled, through increasing gain settings as the gain of the first amplifier stage is increased from a minimum to a maximum gain, a second amplifier stage ( 201 ) can then be enabled in addition to the first amplifier stage and the gain of the second amplifier stage increased from a minimum to a maximum gain, further amplifier stages are enabled as available up to a maximum gain setting for the amplifier array. Each amplifier stage that is enabled has a decreasingly attenuated input signal and a final amplifier stage to be enabled has a full input signal ( 203 ).

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 11/734,864 filed on Apr. 13, 2007, now pending, which is acontinuation of U.S. patent application Ser. No. 11/096,854 filed onApr. 1, 2005, now U.S. Pat. No. 7,250,814, both of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to the field of variable gain amplifiers. Inparticular, the invention relates to variable gain amplifiers for lowvoltage and high frequency operation.

2. Description of the Prior Art

A variable gain amplifier (VGA) is often used in automatic gain control(AGC) applications and often needs to cover a gain range where it isattenuating, with a gain of less than 1, and amplifying, with a gain ofgreater than 1. Modern integrated circuit applications for VGA circuitsare requiring increased operating frequencies, low supply voltages andrelatively high input signal levels (at the low gain settings).

U.S. Pat. No. 6,512,416 to Burns et al. discloses a variable gainamplifier for use in a cable television system. An extended range ofvariable gain amplifier is achieved switching in parallel amplifiers inan architecture. There is no gain adjustment within the individualamplifier blocks of the architecture, the gain adjustment is achieved byswitching amplifiers in and out of circuit. There is mention of varyingthe gain by adjusting the tail current which has several disadvantages,notably the maximum output swing is also changed and the frequencyperformance varies a lot.

The '416 patent also requires a large number of amplifier blocks whichresults in a high capacitive loading at the summing node which requiresthe addition of inductors. The need to use such components on a chip isundesirable due to the large silicon area that they occupy, increasingthe cost of the solution.

Serial link technology is becoming pervasive in the majority of systemto system, system to peripheral or local inter computer connections,replacing parallel bus connections. A variable gain amplifier is animportant part of serial link technology and a solution is particularlyrequired for high frequency applications where high input voltages canbe processed with the low supply of sub-micron CMOS (complementary metaloxide semiconductor) integrated circuit processes. Therefore, it is anaim of the present invention to provide a variable gain amplifiersolution suited for low voltage and high frequency applications.

SUMMARY OF THE INVENTION

This invention comprises a method and apparatus for varying gain of anamplifier array.

In one aspect of the invention, a method is provided for varying gain ofan amplifier array. A first amplifier stage having an input signal witha first amplitude is enabled. Gain of the first amplifier stage iscontrolled to increase the gain up to a maximum for the first amplifierstage. A second amplifier stage having an input signal with a secondamplitude is enabled. The second amplifier stage is in parallel with thefirst amplifier stage. In addition, the first amplitude of the firstamplifier stage signal is different from the second amplitude of thesecond amplifier stage. Gain of the second amplifier stage is controlledto increase the gain up to a maximum. The outputs of the enabledamplifier stages are summed together.

In another aspect of the invention, an amplifier array is provided. Thearray includes at least two amplifier stages connected in parallel. Eachof the amplifier stages has a gain controller. An input signal for eachamplifier stage is provided, with each of the input signals being ofdifferent amplitude. In addition, an enabler and a disabler is providedfor each amplifier stage. A tool to aggregate outputs of the enabledamplifier stages is provided to obtain an output signal, and a controllaw is implemented for amplitude gain versus a control characteristic ofgain.

In yet another aspect, an amplifier array is provided with an enabledfirst amplifier stage in parallel with an enabled second amplifierstage. A first input signal for the first amplifier stage is providedhaving a first amplitude, and a second input signal for the secondamplifier stage is provided having a second amplitude. The first andsecond amplitudes are different. In addition, a gain controller isprovided for each amplifier stage. A output signal is producedreflective of each output of each of the enabled amplifier stages.

In an even further aspect, an article of manufacture is provided. Thearticle includes an amplifier array with at least two amplifier stagesconnected in parallel. Each of the amplifiers stages has a gain controlmeans, with at least one of the gain control means being a digital gainvector to obtain an output. In addition, input signal means are providedfor each amplifier stage with each input signal of each of the amplifierstage having a different amplitude. Means are also provided for enablingand disabling each of said amplifier stages, and for summing output ofthe enabled amplifiers stages to obtain an output signal.

Other features and advantages of this invention will become apparentfrom the following detailed description of the presently preferredembodiment of the invention, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a variable gain amplifier inaccordance with the present invention;

FIG. 1B is a graph showing the gains of amplifier stages in a variablegain amplifier in accordance with the present invention;

FIG. 2 is a diagram of an implementation of a variable gain amplifier inaccordance with the present invention;

FIG. 3 is a diagram of an attenuated signal amplifier as provided in thevariable gain amplifier implementation of FIG. 2;

FIG. 4 is a diagram of the gain network for the attenuated signalamplifier of FIG. 3;

FIG. 5 is a diagram of the full signal amplifier as provided in thevariable gain amplifier implementation of FIG. 2;

FIG. 6 is a diagram of a mechanism for enabling and disabling the fullsignal amplifier of FIG. 5;

FIG. 7 is a diagram of a gain decoder as provided in the variable gainamplifier implementation of FIG. 2; and

FIG. 8 is a diagram of a filter as provided in the variable gainamplifier implementation of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A basic variable gain amplifier (VGA) architecture 100 is shown in FIG.1A. The VGA 100 employs a multipath amplifier arrangement. A firstamplifier 101 uses a full size input signal 103 and a second amplifier102 uses an attenuated form 104 of this input signal 103. A resistorinput network 105 is used to attenuate the input signal 103 to providethe attenuated form 104 for the input signal to the second amplifier102. In place of a resistor input network, a line termination or aparallel divider network could be used.

Each of the first and second amplifiers 101, 102 has a gain adjustmentmeans 107, 108 which is controlled by a gain control network 106. Theoutputs 113, 114 of the first and second amplifiers 101, 102 are summedby summer 110 to provide the overall output 112 of the VGA 100.

At low gain settings which cover the gain range where the VGA 100 isattenuating, the second amplifier 102 using the attenuated form 104 ofthe input signal 103 is active with the first, full signal amplifier 101disabled.

As the gain of the VGA 100 is increased, the gain of the secondamplifier 102 is increased through the gain control network 106. Whenthis second amplifier 102 reaches its maximum gain setting, the first,full signal amplifier 101 is enabled (with its gain set to minimum) andits output 113 is summed 110 with the output 114 of the second amplifier102 thereby increasing the output 112 of the VGA 100.

During the remainder of the control range for the VGA 100, the gain ofthe first amplifier 101 is increased up to its maximum value with thegain of the second amplifier 102 remaining at its maximum.

This arrangement is scalable with it being possible to add moremultipath amplifiers which operate off a range of attenuation outputsfrom an input network.

FIG. 1B is a graph showing the cumulative gains of a plurality ofamplifiers such as the first and second amplifiers 101, 102 of FIG. 1A.

FIG. 1B shows a first amplifier 121 with an attenuated gain 131 startingfrom a gain of zero and increasing to a maximum gain 132. A secondamplifier 122 is then used in addition to the first amplifier 121 andthe gain 133 of the second amplifier 122 is added to the maximum gain132 of the first amplifier 121. The gain 133 of the second amplifier 122is increased until it reaches a maximum gain 134 for the secondamplifier 122. In an embodiment which extends the number of amplifiersfrom the two shown in FIG. 1 a, a third amplifier 123 and subsequentamplifiers can be added. In this way a wide range of gain fromattenuation to amplification is enabled.

FIG. 1B shows a constant gradient for the gain as the multipleamplifiers 121, 122, 123 are added; however, in practice due to thevarying attenuation of the input signals for the amplifiers, this maynot be constant. For example, a more gradual increase in gain over theattenuation range for low gains may be provided with an attenuated inputsignal for the first amplifier 121.

FIG. 1B shows a linear control law for the gain. This control law couldtake other forms as long as the control law is continuous.

This arrangement has several advantages in that it allows operation withhigh input signals at low supply voltages providing that in the highinput case the VGA is operating within its attenuation region of gain,which is often the case. Under these circumstances, the active amplifieris not receiving the full input swing facilitating operation at lowvoltages.

The parallel amplifier topology is also suited to high frequencyapplications where effects such as output jitter can be significant.Instead of cascading amplifiers in series which is well known to bedetrimental to jitter and bandwidth, the multipath arrangement providesincreased gain without this degradation. The extension of multipathamplifier design to VGA applications allows the input operating range tobe conveniently extended for a given supply voltage. The circuit powercan be proportional to the VGA gain and use of parallel stages can easethe implementation of gain control and the minimisation of glitches asthe gain is adjusted.

A practical implementation of this system is shown in FIG. 2. In thiscase the multipath VGA 200 is comprised of two channels, the firstoperating off a full size differential input signal 203 formed by AP/ANand the second operating off a half size input signal 204 formed byAP2/AN2. These signals 203, 204 can be conveniently derived from aresistor input network 105 as shown in FIG. 1A.

Two differential amplifiers 201, 202 provide a current output into aresistor load that can be conveniently connected together to implementthe summer function 10 of FIG. 1 producing the VGA outputs 212 of ZP andZN.

The reference for the current sources used to define the tail current227, 228 in the amplifier circuits is provided by bias block 217 whichuses a reference input voltage VB 215. This bias line is controlledusing a current switch 216 into the full signal amplifier 201 such thatthis channel is only active above certain gain settings of the VGA gain.

The current switch 216 steers the current between the two amplifiers201, 202. When a high gain is required the full signal amplifier 201 isenabled by providing tail current 227 to that amplifier 201. When thefull signal amplifier 201 is not active the current switch 216 steerscurrent 228 into the output of the other amplifier 202 in order tomaintain a common mode output.

The VGA 200 has common power supply terminal voltages, the positivesupply voltage VDD 229 and the negative supply ground VSS 230.

The ability to power down the VGA using the input PDWN 225 is alsocontrolled through this block 217. A separate low noise supply AVXX 226is used in this embodiment for the amplifiers 201, 202 to minimise thenoise on the signal path.

In this case the VGA gain is set digitally through a four bit gainvector DIG<3:0> 214. This vector 214 is decoded such that the mostsignificant bit is used to switch in the full signal channel.Consequently for gain settings from 0000 to 0111 only the half signalchannel is active. At 1000 the full signal channel (at its lowest gainvalue) is added to the output of the half signal channel which is nowmaintained at its maximum gain for the remaining VGA gain settings inthe range 1000 to 1111. The three least significant bits of the gainvector 214 are fully decoded to control the individual amplifiers 201,202 using the thermometer decoders 218 and 219.

In this implementation, a first decoder 218 controls the full signalamplifier 201 and the second decoder 219 controls the half signalamplifier 202. In a first range of the VGA 200, the second decoder 219controls the half signal amplifier 201. During this phase the fullsignal amplifier 201 is not enabled and therefore the first decoder 218is not active or is disregarded. When the second decoder 219 reaches amaximum output, a second range of the VGA 200 is entered. The fullsignal amplifier 201 is enabled and the first decoder 218 drives thefull signal amplifier 201. The second decoder 219 is maintained at itsmaximum output during the second range.

The outputs of the first decoder 218 for the full signal channel areANDed 222 with the most significant bit of the gain vector 214 such thatthe decoder outputs are only active for the channel when this bit isset. This enables the lower 3 bits of the 4 bit gain vector to controlthe full signal amplifier channel when the most significant bit of thegain vector is set to 1.

The outputs of the decoders are filtered using filters 220, 221 beforecontrolling the amplifiers 201, 202. This minimises the disturbance onthe output of the VGA 200 as the gain is increased or decreased betweenadjacent settings of the gain vector 214.

The two decoders 218, 219 could be implemented as a common decoder withfurther gating provided at the output. Many different implementations ofthe decode function are possible.

Gain control signals 223, 224 in the form of digital vectors DIG<6:0> asoutputs 223, 224 from the decoders are input to the amplifiers 202, 201via the filters.

In an alternative implementation, the gain control can be provided byequations implemented into the logic of an integrated circuit ashardware description language code.

The schematic of the half signal channel amplifier 202 of FIG. 2 isshown in FIG. 3. This is a conventional CMOS (complementary metal oxidesemiconductor) based differential amplifier using source degeneration tocontrol the gain. The half signal channel amplifier 202 has a gaincontrol 300 controlled from the decoder outputs 223. FIG. 4 shows anetwork 400 suitable for controlling the gain.

Each decoder output DIG<6:0> connects an impedance between the sourcesof the amplifier input devices 402, 403 at I_N and I_P. The decoder isessentially a thermometer decoder and as the gain is increased more ofthe decoder outputs go high switching in additional elements of thenetwork 400 and the impedance across I_N and I_P reduces, therebyincreasing gain. FIG. 4 shows seven states 411, 412, 413, 414, 415, 416,417 for gain settings DIG<0> to DIG<6>.

Conversely as the gain is decreased then elements are switched out ofthe network 400 increasing the overall impedance and reducing theamplifier gain. The gain of the amplifier 202 is a complex function ofthe gain network impedance and by having an impedance which is specificto each gain setting (as shown by the seven states in FIG. 4), thecontrol law of the amplifier 202 can be set to the requiredcharacteristic. The control law is defined as the amplifier gain versusgain vector setting characteristic.

Another advantage of this arrangement is the incremental change in thenetwork 400 between successive gain setting with just one element beingadded or removed from the circuit. This minimises the disturbance on theoutput as the gain of the VGA is incrementally changed which isdesirable in certain applications. It is well known that the highfrequency performance of the differential amplifier can be improved byoptimising the capacitive loading across a degeneration network and therelative position of the FET (field effect transistor) and resistorwithin each element of the network and their individual sizings can beoptimised for amplifier bandwidth.

The schematic of the full signal amplifier 201 of FIG. 2 is shown inFIG. 5. This uses a similar approach having a differential amplifierwith source degeneration and a gain control 500 controlled from thedecoder outputs 224. A network similar to that shown in FIG. 4 is usedto control the gain, the only difference being in the values of theindividual elements within the network 400.

However, since this channel is only active above a certain value of thegain vector, this amplifier 201 can be disabled using the circuit 600shown in FIG. 6 which is an implementation of the current switch 216 ofFIG. 2.

The circuit 600 is a long tail pair switch which essentially steerscurrent from the tail current source to the load of the amplifier andvice-versa as the amplifier 201 is disabled and enabled respectively.

In this way, the common mode output of the VGA is maintained. This isimportant on low voltage designs where large common mode variation isnot possible.

Since the summing function on the output of this multipath amplifierarrangement is performed by connecting together the current outputs ofeach differential amplifiers 201, 202, it becomes even more important tomaintain the common mode voltage at the output as amplifiers areswitched in an out. It is well known that large changes in the commonmode voltage output of an amplifier can have a detrimental effect on itsown performance and that of successive stages.

An implementation 700 of the decoder 218 of FIG. 2 is shown in FIG. 7.In this embodiment, a 3 bit wide digital vector 701 is converted into 7control lines 702 for the network 400 shown in FIG. 4. With the 3 bitvector at 000, all outputs are at 0 but the gain network has anunswitched element (reference 404 in FIG. 4) which provides the minimumgain setting. As the gain vector moves from 001 to 111 the outputsD<6:0> progressively turn on starting with 0000001 and finishing at1111111. The design of such decoders is well known and FIG. 7 representsone possible implementation 700.

Filters 220, 221 are used between the decoders 218, 219 and theamplifier gain control networks 400 to avoid any glitches from thedecoder 218, 219 coupling into the signal path of the VGA. A schematicfor a filter 220 is shown in FIG. 8 and essentially this is just a bankof resistors 801 which are inline with each decoder output. The decoderhas an input 802 and an output 803 into and out of the filter 220. Theresistors 801 form a low pass RC filter working with the inputcapacitance of the gain control network 400 which is predominantlyformed by the input gate capacitance of the switch FET (field effecttransistor). Again this filter 220 is implementation specific, dependingon the design of the decoder and the performance specification it maynot be required.

The described embodiments provide a variable gain amplifier in which nmultiple amplifier stages are used with a range of fractional inputsignals. Specifically described are two amplifier stages, one with afull signal and one with an attenuated half signal. This is an optimumarrangement for one application in which a very high bandwidth must beaccommodated.

The described arrangement is scaleable using a range of fractionalinputs to drive the inputs. Different ratios of signals may also beused. It is envisaged that other applications may require multiples ofthe full signal to be used for one or more amplifier stages. It is alsopossible for the amplifier stages to all have attenuated signals with nofull signal input, if an application has this requirement. It is alsopossible that one amplifier stage can have a fixed gain with one or moreother amplifier stages having variable gain. All these variations to thedescribed embodiment fall within the scope of the present invention.

The described embodiments of the amplifier array have an amplifieroutput with a voltage output with a common mode of voltage maintained.The described arrangement could equally be applied to an amplifier arraywith a current output, for example a trans-conductance amplifier, inwhich an average current is maintained at the output.

The described variable gain amplifier has the following advantages overknown devices.

At minimum gain which implies maximum input signal, the input stage ofthe VGA does not have to handle the full input swing which can be aproblem with low voltage designs. An input attenuator is used to reducethis signal for the first amplifier in the multipath arrangement.

As the gain is increased, parallel amplifiers are summed together toprovide the VGA output, in this way the gain of the VGA can be greaterthan an individual amplifier. Although a similar result can be obtainedby cascading amplifiers in series, a parallel combination can provideadvantages over this arrangement in terms of distortion and reduction inthe jitter introduced on the signal passing through the VGA.

The parallel approach with parallel amplifiers being switched in or outof the signal path as the gain is respectively increased or decreasedcan be an advantage in reducing the disturbance that occurs on the VGAoutput as the gain is progressively changed. Within many AGCapplications, it is important to minimise the disturbance on the VGAoutput, which could be in the form of signal glitches, for example, asthe gain is changed from one setting to the next. In the multipatharrangement the parallel amplifiers can be slowly switched in or out ofcircuit as the gain range is traversed with minimal disturbance on theoutput.

The implementation of the gain control network by the progressiveaddition or subtraction of elements through a parallel or seriescombination is advantageous. In this way the disturbance on the totalnetwork impedance and hence circuit output can be minimised betweenadjacent gain control settings. In the scheme described this is achievedusing a thermometer decoder on the digital gain control to controlparallel elements in the gain control network. This is a desirablefeature in many VGA applications, for example, an AGC loop, where gainis typically swept between the initial and final values and the outputshould smoothly follow the gain changes without any other disturbancesuch as signal glitches.

The described system also has the ability to use the configuration ofthe gain control network to implement a chosen control law for the gainversus control setting characteristic. Since the network value isautonomous for each setting, the only constraint is that it iscontinuous, the control law can be tailored to the requiredcharacteristic. For example, a linear control law may be required inwhich the gain changes linearly with respect to the settingcharacteristic. As another example, the control law may be logarithmic.The control law can be any form of a curve which is continuous in thatthe change is progressive with no reverses.

Another advantageous feature is the optimisation of the gain controlnetwork such that the bandwidth of the amplifier is also optimised usingthe parasitic capacitance associated with the network to introducepeaking in the amplifier. In this way, the peaking contribution can alsobe changed for individual gain settings.

This arrangement is suited to low power solutions since the totalcircuit power could be reduced with decreasing gain with the unusedparallel amplifiers being disabled. In this way the power could be madeproportional to gain.

The method of disabling an amplifier channel in a multipathconfiguration by steering current between the source and drain of theinput devices of the amplifier is also beneficial. An embodiment of thisprinciple is shown in FIGS. 5 and 6. This allows the common mode at theoutput of the amplifier to be maintained which is important for manyapplications. It is especially desirable in the multipath amplifierconfiguration as it allows multiple amplifier outputs to be directlyconnected together without the common mode of the combined outputchanging significantly as individual channels are switched in and out.

Finally, the VGA scheme described can employ either digital or analogcontrol of gain. Although the implementation described uses digitalcontrol, a linear scheme could be simply implemented using lineardegeneration networks at each amplifier stage. Linear control of theswitching in and out of the parallel amplifiers is also feasible usinglinear control of the current switch. However, in many applications whenlinear control is required this is a ‘fine’ adjustment of a main digitalcontrol, this could be easily implemented with a simple lineardegeneration at an amplifier stage.

Improvements and modifications can be made to the foregoing withoutdeparting from the scope of the present invention.

1. An amplifier array comprising: at least two amplifier stagesconnected in parallel; each amplifier stage having a gain controller; aninput signal for each amplifier stage, each of said input signals of theamplifier stages being of different amplitude; an enabler and disablerfor each amplifier stage; a tool to aggregate outputs of said enabledamplifier stages to obtain an output signal; and a control law adaptedto be implemented for amplitude gain versus a control characteristic ofgain.
 2. The amplifier array of claim 1, further comprising maintenanceof a common mode at an output of said amplifier array.
 3. The amplifierarray of claim 2, wherein maintenance of said common mode includes steerof current between a source and a drain of said amplifier stage.
 4. Theamplifier array of claim 1, further comprising a total gain of saidamplifier array having a range from a low gain setting with said firstamplifier stage enabled to an increased gain setting as gain from saidfirst amplifier stage is increased from a minimum gain to a maximumgain.
 5. The amplifier array of claim 1, further comprising said secondamplifier stage adapted to be enabled in addition to said firstamplifier stage, and gain of said second amplifier stage having a gainincreased from a minimum setting to a maximum setting.
 6. The amplifierarray of claim 1, wherein said first amplifier stage having a mostattenuated input signal for said low gain setting.
 7. The amplifierarray of claim 1, wherein each amplifier stage includes a decreasinglyattenuated input signal and a final enabled amplifier stage having afull input signal.
 8. The amplifier array of claim 1, wherein a value ofsaid gain control is autonomous for each characteristic and said law iscontinuous.
 9. The amplifier array of claim 1, wherein said gain controlof an amplifier stage may optimize bandwidth of said amplifier arraythrough an introduction of peaking in said array and to support a changein peaking for each gain setting.
 10. The amplifier array of claim 1,wherein said input signal includes means adapted to increase attenuationof said input signal to said amplifier stages.
 11. The amplifier arrayof claim 10, wherein said input signal means is selected from a groupconsisting of: a parallel divider network, and a line termination. 12.The amplifier array of claim 1, wherein said amplifier stage is adifferential amplifier and said gain controller of each amplifier mayinclude a source degeneration.
 13. The amplifier array of claim 1,wherein the gain controller gain of said amplifier stages is adapted toinclude a process selected from the group consisting of: optimization ofbandwidth of an associated amplifier, source degeneration, andprogressive addition and subtraction of elements through a parallel andseries combination to change impedance of a network.
 14. The amplifierarray of claim 1, wherein said gain controller is a digital gain vectoradapted to obtain an output.
 15. The amplifier array of claim 14,further comprising a filter adapted to parse said output to minimizedisturbance on said output as gain is changed between settings of saidgain vector.
 16. An amplifier array comprising: at least two amplifierstages connected in parallel; each amplifier stage having a gaincontroller, with at least one of said amplifier states having a mostattenuated signal for a low gain setting; an input signal for eachamplifier stage, each of said input signals of the amplifier stagesbeing of different amplitude; an enabler and disabler for each amplifierstage; and a tool to aggregate outputs of said enabled amplifier stagesto obtain an output signal.
 17. The amplifier array of claim 16, whereina gain controller of said first amplifier stage is adapted to increase again of said first amplifier stage to a maximum and a gain controller ofsaid second amplifier stage is adapted to increase an associated gain upto a maximum.
 18. An article comprising: an amplifier array having atleast two amplifier stages connected in parallel; each of said amplifierstates having gain control means, wherein at least one of said gaincontrol means is a digital gain vector adapted to obtain an output;input means for each amplifier state; input signals of each of saidamplifier state being of different amplitude; means for enabling anddisabling each of said amplifier stages; and means for summing output ofenabled amplifier stages to obtain an output signal.